Magnetic head having a continuously variable radius of curvature

ABSTRACT

A magnetic head employing a fluid bearing between the head operating surface and a tape moving thereover, with the head surface configuration characterized by a continuously varying radius of curvature.

United States Patent Friedrich R. Hertrich;

Sanford Platter, both of Boulder, Colo. 783,696

Dec. 13, 1968 June 1, 1971 International Business Machines CorporationArmonk, N.Y.

Inventors App]. No. Filed Patented Assignee MAGNETIC HEAD HAVING ACONTINUOUSLY VARIABLE RADIUS OF CURVATURE 6 Claims, 8 Drawing Figs.

US. Cl 340/1741, 179/ 100.2

Int. Cl Gllb 5/60 Field of Search 340/174.1

E, 174.1 F; 179/1002 P, 100.2 C

[56] References Cited UNITED STATES PATENTS 3,398,870 8/1968 Mullen etal 340/174.1 3,416,148 12/1968 Berghaus et al. 340/1741 3,416,14912/1968 Stahler 340/] 74.1 3,475,739 10/1969 la Manna 340/ 1 74.1

Primary ExaminerTen-ell W. Fears Assistant Examiner-Vincent P. CanneyAtrorneySughrue, Rothwell, Mion, Zinn and Macpeak ABSTRACT: A magnetichead employing a fluid bearing between the head operating surface and atape moving thereover, with the head surface configuration characterizedby a continuously varying radius of curvature.

PATENTEU JUN n97: 3,582,917

sum 1 or 2 FIG. I

PRIOR ART INVENTOR FRIEDRICH R. HERTRICH SANDFORD PLATTER ATTORNEYS.

PATENTEU JUN 1 I9?! SHEET 2 OF 2 FIG. 7

.INVENTOR5. FRIEDRICH R. HERTRICH SANDFORD PLATTER AZM FIG. 8

ATTORNEYS,

MAGNETIC HEAD HAVING A CONTINUOUSLY VARIABLE RADIUS OF CURVATUREBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to magnetic transducer apparatus and more particularly toflexible record media transducing head of a type employing aself-generated fluid bearing between the moving record media and thetransducing operating head surface for supporting the record mediaspaced slightly from the transducing head.

2. Description of the Prior Art When a flexible record media such as amagnetic tape is moved at some velocity over the operating surface of atape read or write head, a hydrodynamic air film is formed between thetape and the head surface, which acts as a fluid lubricantf The gapexisting between the moving tape and the head surface should,necessarily, be maintained as small as possible to ensure maximumtransducing efficiency while at the same time sufficiently large toprevent wear on both the tape and its head surface. The hydrodynamicfluid film which is created due to relative velocity between the tapeand the head, tends to maintain the tape out of contact with the headsurface. The parameters involved in the maintenance of a controlled airfilm between the tape and head surfaces are: the shape of the head, thevelocity of the tape, the tension acting on the tape, the angle at whichthe tape is wrapped about the head, and the viscosity of the fluid. Therelationship between these parameters is set forth in U.S. Pat. No.3,170,045 to Heard K. Baumeister et al. and assigned to the commonassignee.

The magnetic head of that patent, in one form employs an operatingsurface having a constant radius of curvature with the tape partiallywrapped thereabout under applied tension. The thickness or spacing ofthe fluid bearing is detennined by the formula:

h* T 0.642- R where h is the air film thickness, T is the tape tension,p, is the viscosity of the fluid medium (normally air), V is the tapevelocity and R is the radius of the curved working surface of the head.

Since the radius of curvature of a transducer head of this type isconstant, the air bearing thickness is determined by asingle radiusvalue. This thickness is maintained constant only if the fluid bearingexists over an appreciable arc length along the operating surface of thehead. If the arc length is too short, it is impossible to maintain aconstant fluid bearing over the surface of the head structure. Thisaffects the wear properties of the tape and the head operating surface,as well as the transcribing quality of the apparatus. Sufficient arclength may be achieved by employing large radii or by wrapping the tapeat substantial wrap angles over small radii. This, however, is not asatisfactory solution in all cases since, problems exist where largeradii are used, since the tape to head separation h* is directlyproportional to radius as set forth in the above formula.

Attempts have been made to solve this problem by providing a transducerwhose operating surface has curved portions including at least the entryand transducing portion having different radii of curvature. The radiusof curvature of the entry portion is generally smaller than the portionof the structure containing the transducing gap itself. A structure ofthis type is set forth in copending patent application, Ser. No.420,602, entitled Compound Radius Transducer Head, filed Dec. 23, 1964,to Donald G. Berghaus et al. and assigned to the common assignee. Thisarrangement provides a small constant fluid bearing over the entiretransducing portion of the head with reduced tape and operating surfaceand it is particularly useful in transducing systems where it is notpossible to provide a large wrap angle for the flexible record media.However, the employment of compound surfaces of different radii causesthe contour to go from one radius into another radius and back with anabrupt change causing a dynamic jerk due to the step change ofcurvature. Hence to produce washboard effect on the variations in thepressure and the air film which are abrupt resulting in tape flutter andwhich both affects recording performance, and wear to the tape and thehead surface.

The same problem of step change in curvature occurs where a singleradius of curvature is employed for the transducing portion of theoperating surface with adjoining portions of the working surface areflat. Normal forces acting on the tape by the self induced or appliedfluid film, do not exist in the flat section. As a result, very weakrestoring forces exist to control the desired spacing prior to passingof the tape over the relatively short cylindrical portion carrying thetransducer means. Air bubbles may form under dynamic conditions whichupset the head to tape relationship at the transducer gap and also,damping of transverse oscillations (up and down) of the tape isinsufficient which greatly affects recording and/or reading performance.

SUMMARY OF THE INVENTION This invention is directed to the magnetictransducer head having a curved operating surface whose radius ofcurvature is continuously varying such that no step change in curvatureoccurs. The surface is continuous up to the second derivative and it hasthe desired radius of curvature in the gap region, the required entranceand exit angle and the required radius of curvature in the entrance,exit and central regions. The surfaces may be self-lubricating underhydrodynamic air lubrication principles or externally induced by theapplication of pressurized fluid. Further, vacuum pressure may beemployed for reducing either the applied or self-induced air film in thevicinity of the transducer gap or gaps.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representationof a prior art, single radius, curved magnetic transducer head employedwith a hydrodynamically air lubricated magnetic tape.

FIG. 2 is a schematic representation of a prior art curved magnetictransducer head having curved compound surfaces with the magnetic tapesupported by a hydrodynamic air bearmg.

FIG. 3 is a schematic representation of a prior art, compound radius,air bearing head with curved operating surface portions separated byflat surface portions.

FIG. 4 is a plot of the slope of the transducer head operating surfaceof the compound radius head of FIG. 3..

FIG. 5 is a plot of the curvature of the operating surface of thetransducer head shown in FIG. 3.

FIG. 6 is a plot of curvature of the head surface of a proposedtransducer head whose surface is characterized bya continuous secondderivative of the same.

FIG. 7 is a plotof the slope of the transducer head operating surface ofthe transducer of the present invention whose curvature is shown in FIG.6.

FIG. 8 is a schematic sectional representation of the transducer head ofthe'present invention with the operating surface conforming to the slopeof FIG. 7, and the curvature of FIG. 6.

DESCRIPTION'OF THE PREFERRED EMBODIMENT Referring to the drawings, priorto discussing the structural aspects of the curved operating surfacetransducer head of the present invention, reference may be had to FIG. 1which illustrates schematically a transducer head employing a singleradius, curved operating surface over a moving magnetic tape overlyingthe same and supported by a self-induced or hydrodynamic air bearing.This type of head is shown in theaforementioned US. Pat. No. 3,170,045.The transducer head 10 is characterized by a radius R of constant valuewith the transducer gap 12 centrally located with respect to the entryand exit portions of the transducer head operating. surface.

The magnetic tape 16 in moving, in the direction of the arrow, at highspeed from the supply reel 18 to the takeup reel 20 results in aself-induced air film existing between operating surface 14 of thetransducer head and the magnetic tape 16. Applied tension and thepositioning of the reels I8 and 20, provide an angle of wrap which isshown as being equally divided on either side of the gap 12. The Figureshows a tape wrap of about to each side of the gap.

Turning next to FIG. 2, there is shown a prior art magnetic transcribinghead design characterized by an operating surface composed of compoundradii, of the type described in the pending application entitledCompound Radius Transducer Head," previously referred to. In this case,the magnetic transducer at 110 comprises a read portion for instance 112and a write portion 114 separated by a section 116 located centrally ofthe head. At the transducer gaps 118, the magnetic tape 120 is separatedfrom the operating surface 122 of the transducer head by an air bearingfilm which exists between these elements. The spacing of the magnetictape which moves, under tension in the direction of the arrow, fromsupply reel 124 to takeup reel 126 is defined by the entry region 128having a relatively small radius R,. The self-induced or applied airbearing is maintained between the operating surface of the head and thetape in the large radius region 130 of the transducing gap area 118. Asecond relatively small radius portion 132 is found at the exit side ofthe magnetic transducer head. Portion 132 has a small radius R which maybe equal to radius R,.

While the desired fluid bearing is created at the entry area 128 and ismaintained over the transducing area 130, and while this reduces wear ofboth the head structure and record media and maintains substantiallyconstant transducing signals, at the point where the contour changesfrom one radius such as R, to a different radius such as R there resultsa sudden change in the forces acting on the tape and on the headresulting in unstable operation and wear in this area. Further, sincethe operating surface portions 134 and 136 adjacent the entry and exitportions 128 and 132 are nominally flat, and since the central ortransducing portion 130 is of a relatively large radius, these surfacesdo not generate the necessary normal force required to control thedesired spacing prior to and subsequent to passing the relatively shortportions of small radius at R, and R Air bubbles may form under dynamicconditions which upset the head to tape relationship at the transducinggap area 118. Further, self-damping of the transfer oscillations (up anddown) by the tape tension means is insufficient.

Perhaps the most simplified illustration of the disadvantages of thegenerally compound radius self-lubricated transducer head may be seen byreference to the schematic representation of FIG. 3. In this case,magnetic transducer head 210 is provided with an upper operating surfaceover which lies a magnetic tape (not shown) moving at a given velocity,under a desired wrap angle and spaced slightly therefrom by aself-induced or applied fluid bearing. The operating surface 212 ischaracterized by a flat portion 214 from the leading edge 216 to a pointdesignated X,, a curved entry surface portion 218 between X and X,, andat a given radius of constant value, a flat transducing portion 220between X, and X, and a curved exit surface portion 222 between X, and Xagain at a given radius (preferably equal to the radius of section 218)and, finally, a trailing surface portion 224 which is flat, between Xand the trailing edge 226 of the transducer head.

Reference to FIG. 4 shows the plot of the slope of the typical compoundradius head of FIG. 3. A constant, slope at a given angle exists fromleading edge 216 to point X,. The slope increases from X, and X, andremains constant for flat transducing section 220. The slope thenchanges rapidly from X, to X, for exit section 222 and remains constantbetween X, and the trailing edge 226. In practical design, the curvatureof the head surface is a function of the derivative of the slope,specifically curvature by definition is the second derivative of thesurface divided by the quantity 1 plus the first derivative squared tothe L5 power, and the plot of the same is shown in FIG. 5. The curvatureK which is equal to I over the radius of curvature, shows zero curvaturebetween leading edge 216 and the curved entry section 218 at X,. Thecurvature remains constant at a set value between X, and X with both,this curvature and that for the exit portion 222 between X and Xchanging in step fashion forming an abrupt line of discontinuity betweenflat sections 214 and 220 for entry section 218 and between flatsections 220 and 224 for exit section 222.

The present invention is directed to a continuously variable radiuscontour for the surface of the magnetic transducer head which permitstransition from a very large radius in the entrance region to a smallradius at the recording gaps. By avoiding step changes of curvature, thedynamic jerk experienced by the tape becomes small, the tape conformsmore closely to the head contour, and variations of pressure in the airfilm are less abrupt. Changes of "washboard effects are minimized withan initial contour approximating natural bending curves, and head wearis less likely to cause secondary contour changes. Recording performancemay therefore be maintained over a longer life span.

The present invention is directed particularly to a magnetic transducerhead of this type which carries two transducer gaps which are spacedapart. In this case, the head is symmetrical about a point intermediateof the two gaps, and for each gap, the minimum radius occurs at the gaplocation.

Reference to FIG. 6 shows a function of the second derivative of thehead surface of the present invention wherein the curvature K iscontinuously varied with the configuration shown having the desiredradius of curvature between surface portion X and X in the gap region,required entrance and exit angles in the surface portions between X, andX and X and X and the required radius of curvature in the entrance, andcentral regions of the transducer head operating surface. With thevariable radius contour as shown by the plot of the curvature in FIG. 6,transition from a very large radius in the entry region to a smallradius at the recording gap is readily achieved. Normal loading of thetape may start at a low level which gradually increases to a maximumload at the gap between X, and X As a result, the tape to head spacingis well controlled in the entrance region, in the transition region andin the operation region. For instance, in the entrance region, head totape spacing reduces to 100 microinches, in the transition region thespacing reduces from 100 microinches to a dimension on the order of 20to 40 microinches and in the operation region spacing is preferablymaintained at 20-40 microinches. A high normal load at the gaps isessential for maintaining a stiff air bearing and this is accomplishedreadily by the head surface of the present invention which ischaracterized by small radii in the area of the gap while in the areabetween the gaps, the normal load and associated wear exposure isdeliberately decreased by transition to larger radii. In fact, thedistribution of normal load and indirectly the head to tape spacing maybe more closely controlled with heads employing the present headoperating surface configuration than in the prior art compound radiusheads. It is noted for instance that, in a given example, the largerradius of curvature at the entry and exit region of the transducer whichmay be in the order of 20 inches, merges into a 1-inch radius at thetransition gap and then into a 5-inch radius at the center of the head.Of course the radii continuously change to blend a series of radii intoone continuous surface. It may be stated, that the transducer head ofthe present invention is characterized by having l a curved surfaceconfiguration such that there is no step change in curvature along itscomplete length, (2) at any given point along its length there is but asingle radius of curvature, and (3) a continuously varying radius ofcurvature which defines the operating surface.

This relationship may readily be seen by further reference to FIGS. 7and 8. While the schematic sectional representation of the head 310 asshown in FIG. 8 is essentially a combetween the prior art configurationand that of the present inpheric pressure, some of the pe leaves throughthe slots or e in head-topneuqually g F IG. 7 where While the proposedmagnetic head 310 of the present inons between X vention operatessatisfactorily without surface modification, the fact that the tape iswrapped over a radius surface head under tension, causes pressure to bebuilt up between the head rma- 5 and the tape. If the tape wereperfectly flexible, this pressure rporating the conwould be equal to thetension divided by the radius of curvature. In typical applications, aradius of curvature of 1.0 inches g f a or larger results in a verylarge head-to-tape separation. It is the X distance indication proposedto reduce this separation by the employment of a seputelgiven 0 ries ofnarrow and shallow slots 312 running perpendicular to mple are as tapemotion to further provide optimized pressure distribution for the fluidbearing. These slots become sinks at atmos pressure (0 p.s.i.g.).Because of the zero flow brought along with the ta 1 5 sinks. Thisresults in a decreas tape separation in Figure 7 Figure 6 the regiondownstream from the sinks. Thus, this has the effect Y Angle slop m ofcontrolling the separation without the use of external matic supplies.This concept of separation control is e vention becomes readily apparentwhen viewin the slope for the transition and operating regi and X, arereadily distinguishable over those for the same re gions in H0. 4. Withreference to FIGS. 6, 7 and 8, the Figures were developed fromcomputer-generated info tion concerning an actual head design inco ceptsof the present invention. It is to be noted that the head is symmetricalabout its center point and startin reference point at the center, X4)increases in either direction therefrom. The com plot points and theparameters relative to this exa follows:

LIST OF POINTS, DIMENSIONS IN INCHES Flgure 8 e r. e ele t g ae a g .gmmm m am mmwa MM m mm mmma m mwdmw mafia D. .l g 8 n a S S or ec c o tod io m e le n nm 0 5 r. d H8 68H 3 .fl m h .OIC d 8 h .0 0a.] s mln m lm m a s eunm mnonx v e t s P oi w le .t w f. e oe rnvd ww h "C C a J mSOMU a n w nm mmn e u mu 3M.e.mu w 0 S e flee... h c d m r c c nu n n Pr m m fifimm m w a fidw mm a mam mwwm m fimm ll .1 8 u m mn a h lseno wfl n u fln e ruu .lte 1.1M

.l r.. n I B t. mu w e w mm m y fi m ommm w mfl m a u c n n 0 h m m m eh n m m o m m n 1 w n n m m a e a m m o m m w m n m. v a a t o r e oht ou im r i i h hi ys c v v c ms d t P mo. ua d t l. l 00 3 [h mam Wm me mpmm fln mem %s 3% m] ma mam c .l.& o mmr a i w s .1 m mst m m Ua cnPruntd d m m.....nude .m m m M m n mmm f s d nokg f m O m .mm 1 0 e.l.m l wfiall Oe m u e t nhonrke er er .1m cy ehfl a nec omw a s fl o me mh be mnm am emn u mm cn mk wht u um. w e nhm mv wcofi h a an. gtwnrs m sd nhmn w a s nie a tgw Pe r 0 n 06 eumt m n u euin nwe n mmnaup m w mn lmtmct pm o mc w ouh ee n ools a nadfi. m md tfi.mu.mbhadde c c c t o w.Pm a h t m m d h m e y s m a t s a n m e r m e e t 1 d w m "a h s r n aav tn mbn d p.. dn m fldm m p.l nm m 0h0 n c a e a d fl O .manr.pfl. mtb t n o n aavdro cvm a ed o W c ha e c a m.l m u b e8 fw fl afi e RVSUC0. oc e r u m mnmm ya mdn P c m es s em a d es tm saalLa bmWf aw .mdme hu c y a n.w c yan nv oecwt ew mt we n .f te n t ca .em r oymmrhdmirtmeieu m uwe a dn k aue m a ll t m wl m h mnfl m t v s mn.nc eh o smP u s lne. t a t a a rrn 1.... n nwn lPe mum mmmmm mmm m m mmmmmwmmmmmm m mmmm wwm wm m w ce,alPr dm mw m f n wm imm m n n wmum wmmw ems t m.Wmn mlmowmmemmt m memn mnmemwo n e an d p uv e p e e ns aeeos .l gpOC e T 0b .1 .l T.lna T .l m. m a a mm m m WM m m m m u w m m2 U ml mmm 4 m u mm 5 mm mm M Ma 6 m m m e Proaoe eio eulha 0h e na m mhe e a m hm m aoflfw vPc dmcP tth ws ire sattr hmtsuoc saufmf w x w u w. e w w w e317 5 7 0 1 74 573323 7460 21 m mnnmmmmmcmnmmmamaawnmummmmmmmmmwmmmmmmmmmmwmmmmmmmmmmmmmmmmmmmmmmmmmaanwuuauanmuuauuuianwaaaunannouncedauumuuumaamuuuumuuaua L0 .u0 nw0 0 00 nw0 n0 0 0 .u0 0 m0 n 1 3 57 0 8 93 3 5 77 99 00 11 7 0123456789 1 7 9mmmm mmmmmmmmmmmmmmmmwwmnmmw mmmmwwmuwmwmwmmm mmmwmmmmmmmmmmmmmm meme mmumaodaa.unwoaaoaunwuanruaaaaoaaaQuadsQuadQuadoaoeuuauuoaaauaaaauuuunwounwdownmammmmmwmmmmmwmmmmmmwwmmwmmmmmmm mmmmmmmmmmmmmmmmsmmmmmmmmmmmmwmmmmmmmmmmmmmmmmmmmmm mmm mmm mm m mm m m m m u m mmmmmmmmwmwmmmmmmm m wmwmmmmmmmmm w mmmmwmmmmmwm mm. 00000000 0 00 Gap posltlom.

therebetween.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,582,917 Dated June 1, 1971 Inventor) Friedrich R. Hertrich et a1 It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

In column 6, line 55- delete"said operating surface,"

(Claim 5) Signed and sealed this 26th day of October 1971 (SEAL) Attest:

EDWA D M.FLETCHER,JR. ROBERT GOTTSCHALK Acting Commissioner of PatentsAttesting Officer FORM PO-10501069) n u s GOVUIMMENY rmmmrornc: nuo-sn-su

1. In a magnetic head having an operating surface adapted to support afluid bearing between itself and a tape moved thereover, the improvementwherein said operating surface has a curvature defined by a continuouslyvarying radius.
 2. The magnetic transducer head as claimed in claim 1wherein said curved operating surface is continuous up to the secondderivative.
 3. The magnetic head as claimed in claim 1 furthercomprising a plurality of spaced transducing gaps with the minimumradius of curvature for said operating surface occurring at each gapthereof.
 4. The transducer apparatus as claimed in claim 3 wherein saidfluid bearing is self-induced by said magnetic tape moving at a givenvelocity over said curved operating surface and said transducer furtherincludes transverse slots formed in at least the transducing portion ofthe head operating surface to reduce the thickness of air film existingtherebetween.
 5. Transducer apparatus comprising; a magnetic transducerhead including at least one transducing gap, means including a movingmagnetic tape defining a fluid bearing between the tape and the headoperating surface, said operating surface, said operating surfaceincluding at least an entry portion and a transducing portion carryingsaid transducing gap with said operating surface defined by a variableradius without a step change in curvature throughout the length thereof.6. The transducer apparatus as claimed in claim 5 wherein said fluidbearing is self-induced by said magnetic tape moving at a given velocityover said curved operating surface and said transducer apparatus furtherincludes transverse slots formed in at least the transducing portion ofsaid head operating surface to reduce the thickness of the air filmexisting therebetween.